[1] Field of the Invention
The present invention relates to an image pickup system in which a plurality of unit cells each performing photoelectric conversion on light incident thereto are arranged on a semiconductor substrate in a one- or two-dimensional array, in particular to a technology for preventing a phenomenon that images become dark due to the entrance of intense light.
[2] Description of the Related Art
Late years, image pickup systems using an image pickup apparatus are commonly used, and representative examples of such image pickup systems are home video cameras and digital still cameras.
Some of these image pickup systems have an amplified image sensor as the image pickup apparatus.
Although having excellent characteristics such as a low noise level, amplified image sensors have a problem that images are subject to darkening due to the entrance of intense light.
In Japanese Laid-Open Patent Application No. 2000-287131 (Patent Reference 1), disclosed are an overview of a CMOS image sensor, which is an amplified image sensor, the same problem described above, and the CMOS image sensor that detects, with respect to each pixel sensor, an input of intense light based on a reset output voltage and replaces the voltage at the reset with another voltage. Patent Reference 1 describes therein that the CMOS image sensor is capable of preventing the aforementioned problem.
In Patent Reference 1, change in output voltages at the reset is used as an index for detecting pixel sensors that will cause image darkening. However, the change in output voltages at the reset is the very cause leading to image darkening, and has a direct effect on luminance information even if the amount of the change is small.
However, the change in output voltages at the reset cannot be detected unless the amount of the change reaches a certain magnitude. Thus, it is difficult to completely eliminate the adverse effect caused by the change in output voltages at the reset.
In addition, the change in output voltages at the reset as a result of an input of intense light is characteristically abrupt and is difficult to be detected with precision. Consequently, it is not an easy task to ensure prevention of image darkening.
For example, when an object taken by the CMOS image sensor, disclosed in Patent Reference 1, has a sufficiently bright central portion and a periphery gradually darkening outwards, it is possible to prevent the sufficiently bright central region on the taken image from darkening. However, in the periphery, a portion close to the boundary with the region prevented from darkening becomes darker although it should be light. Depending on the luminance distribution on the object, a ring-shaped darkened portion may appear.